As information output apparatuses for, e.g., wordprocessors, personal computers, and facsimile apparatuses, printing apparatus which print desired information such as characters and images on printing media such as paper sheets and film sheets are widely used.
Various systems are known as printing systems of such printing apparatus. An inkjet system which prints by discharging ink from a printing means (printhead) onto a printing medium has the advantages that, e.g., a printing apparatus can be readily made compact, high-precision images can be printed at high speed, the running cost is low, noise is low because the system is a non-impact system, and color images can be easily printed by using ink liquids of a plurality of colors. Therefore, this inkjet system is widely used as a general printing system.
In a printhead of a printing apparatus (to be referred to as an inkjet printing apparatus hereinafter) using the inkjet system, discharge orifices (nozzles) have variations in discharge rate and discharge direction. When a plurality of discharge orifice rows are formed, slight variations are produced in accuracy of attachment to the printhead. As a consequence, the printing position of one nozzle row slightly differs from that of another nozzle row. If printing is performed while the relative printing positions of discharge orifice rows are thus different, ruled lines are formed in different positions, or the density of dots printed by ink discharged from the printhead varies, resulting in grainy images.
Accordingly, to improve the quality of printed images, the relative printing positions of nozzle rows must be aligned. This is generally called printing position adjustment.
This printing position adjustment is done by printing, on a printing medium, a plurality of patterns in which the relative printing positions of objects (e.g., nozzle rows) of the printing position adjustment are shifted little by little, and selecting a pattern in which optimum relative printing positions are printed. Methods of selecting the optimum pattern are roughly classified into two methods: a method of allowing a user to select relative printing positions; and a method of aligning relative printing positions by installing a certain relative printing position adjusting means in the printing apparatus itself.
As described above, the printing quality of an inkjet printing apparatus having a plurality of nozzle rows can be improved by adjusting the relative printing positions of these nozzle rows before the printing apparatus is used.
FIG. 8 is a view showing examples of printing patterns for performing the printing position adjustment between a plurality of nozzle rows. This printing position adjustment is performed to adjust the relative printing positions of a plurality of nozzle rows. Accordingly, the type of printing pattern changes in accordance with the type of nozzle row of a printhead. The printing patterns shown in FIG. 8 are printing position adjusting patterns for an inkjet printing apparatus which uses a printhead having an even-numbered nozzle row and odd-numbered nozzle row for each of ink liquids of black, cyan, yellow, and magenta as shown in FIG. 11.
Assume that this printhead shown in FIG. 11 can drive the nozzle rows of the individual colors at respective arbitrary timings without limiting the discharge timings of each color and each nozzle row. Assume also that the interval of the driving timings is so set that dots from the same nozzle can be printed at an interval of 1,200 dpi in the main scan direction during the same main scan.
The printing position adjustment is performed by printing a specific test pattern (printing position adjusting pattern) which allows easy detection of relative printing position differences on a printing medium (generally a paper sheet). On the basis of one nozzle row as an object of the printing position adjustment, a specific pattern is printed a plurality of number of times (in FIG. 8, 11 times from +7 to −3 or from +5 to −5) while the relative printing position of the other nozzle row as an object of relative printing position matching is changed by changing the driving timing. Of these printed patterns, the set value of a pattern having the best matched printing positions is stored in a nonvolatile memory (EEPROM) of the printing apparatus. This process is performed for all nozzle rows (some of them may also be processed together) as objects of the printing position adjustment.
Combinations of nozzle rows to be subjected to the printing position adjustment by using patterns A to F shown in FIG. 8 are as follows.                A: Black even-numbered nozzle row/odd-numbered nozzle row        B: Cyan even-numbered nozzle row/odd-numbered nozzle row        C: Magenta even-numbered nozzle row/odd-numbered nozzle row        D: Black two-way printing        E: Color (cyan) two-way printing        F: Black/color (cyan)        
For yellow, no printing position adjustment is performed between even- and odd-numbered nozzle rows. This is so because the density of yellow is low, and this makes it difficult to determine a set value with which the relative positions match best when the above patterns are printed. For this reason, the result of adjustment of cyan is used for yellow. This cyan adjustment result is also used in two-way printing position adjustment of ink liquids of other colors (magenta and yellow), so no specific patterns for the purpose are prepared.
After the printing position adjusting patterns are thus printed, a set value is selected from the printing results by one of the following two methods. In one method, a user selects a set value from the test pattern printing results, and manually inputs the set value from a host apparatus connected to the printing apparatus. In the other method, the printed test patterns are sensed by an internal sensor of the printing apparatus, and an optimum set value is selected on the basis of a density change or the like.
The printing position adjustment will be described in more detail below with reference to FIGS. 9 and 12 to 15 by taking the pattern A (black even-numbered nozzle row/odd-numbered nozzle row printing position adjusting pattern) as an example.
FIG. 9 is a view showing, in an enlarged scale, the state of dots printed by set value +3 in the pattern A shown in FIG. 8. The abscissa indicates printing positions in the scan direction. Assuming that the scale shown in FIG. 9 is divided for every 1,200 dpi, dots are printed from the left to the right in FIG. 9, i.e., dots are printed in ascending order of value on the abscissa. Blank circles indicate dots printed by an even-numbered nozzle row, and hatched circles indicate dots printed by an odd-numbered nozzle row.
That is, FIG. 9 shows the state printed by repeating a process in which each of an even-numbered nozzle row A and odd-numbered nozzle row B of black nozzle rows 1A of the printhead shown in FIG. 11 is first continuously driven 7 times (7 columns are printed) and then kept undriven 7 times in the main scan direction while the printing position is moved. In this embodiment, printing is performed by moving the printing position by 1,200 dpi at one time. More specifically, dots of the even-numbered nozzle row are printed in main scan direction printing positions 0 to 6 and 14 to 20, and dots of the odd-numbered nozzle row are printed in 10 to 16 and 24 to 30. In main scan direction printing positions 14 to 16, the dots printed by the even- and odd-numbered nozzle rows overlap each other.
FIG. 12 shows the state of those dots of the pattern A shown in FIG. 8, which are printed by set value +2. Similar to FIG. 9, the abscissa indicates printing positions in the main scan direction in which printing is performed, the scale is divided for every 1,200 dpi, dots are printed from the left to the right in FIG. 12, blank circles indicate dots printed by the even-numbered nozzle row, and hatched circles indicate dots printed by the odd-numbered nozzle row. In addition, driving and non-driving of the even- and odd-numbered nozzle rows are switched every 7 times in the same manner as in FIG. 9.
The difference of FIG. 12 from FIG. 9 is that the printing positions of the odd-numbered nozzles are shifted by 1,200 dpi to the left (the driving timings of the odd-numbered nozzles are advanced by 1,200 dpi) without changing printing by the even-numbered nozzles. Consequently, as shown in FIG. 12, although dots printed by the even-numbered nozzle row are formed in main scan printing positions 0 to 6 and 14 to 20 in the same manner as in FIG. 9, the main scan printing positions of the odd-numbered nozzle row are shifted to the left, i.e., to 9 to 15 and 23 to 29. Accordingly, different from FIG. 9, the dots printed by the even- and odd-numbered nozzle rows overlap each other in two main scan printing positions 14 and 15.
FIG. 13 shows the state of those dots of the pattern A shown in FIG. 8, which are printed by set value +1. That is, FIG. 13 shows the state of printed dots when the printing positions of the odd-numbered nozzle row are further shifted by 1,200 dpi to the left from the state shown in FIG. 12 (the driving timings are advanced by the time corresponding to 1,200 dpi). FIG. 14 shows the state of those dots of the pattern A shown in FIG. 8, which are printed by set value 0. FIG. 15 shows the state of those dots of the pattern A shown in FIG. 8, which are printed by set value −1.
As described above, only the printing timings of the odd-numbered nozzle row are changed one after another without changing the driving timings of the even-numbered nozzle row. As a consequence, the main scan direction printing positions of the dots printed by the odd-numbered nozzles change, and this changes the relative printing positions of the dots printed by the even- and odd-numbered nozzle rows. After a plurality of patterns are printed by thus changing the set values, a pattern (i.e., the pattern shown in FIG. 14 of the patterns shown in FIGS. 9 and 12 to 15) in which the dots printed by the even- and odd-numbered nozzle rows most smoothly connect. In this way, a relative printing position set value is determined and stored.
When the pattern shown in FIG. 14 is selected by thus performing the printing position adjustment, if the even-numbered nozzle row is driven at the driving timing when main scan direction printing position 0 in FIG. 14 is printed by the even-numbered nozzle row, and the odd-numbered nozzle row is driven at the driving timing when main scan direction printing position 7 in FIG. 14 is printed by the odd-numbered nozzle row, the interval between the printed dots in the main scan direction printing positions is 7. Therefore, the driving timing of the odd-numbered nozzle row is further advanced by 7 from the state shown in FIG. 14. In this manner, the printing positions of the even- and odd-numbered nozzle rows can be matched in the main scan direction.
As described above, the relative printing position set value of the even- and odd-numbered nozzle rows is determined. This similarly applies to the other patterns (patterns B to F) shown in FIG. 8. That is, on the basis of one of the two nozzle rows as objects of the printing position adjustment, printing is performed by changing the driving timing of the other nozzle row by 1,200 dpi at one time. Consequently, the relative printing positions of the two nozzle rows as objects of the printing position adjustment can be made different from each other. By selecting the smoothest pattern from a plurality of different printed patterns, the printing position set value of these nozzles can be obtained.
When a printhead having a plurality of discharge orifice groups (nozzle groups) is so controlled that different discharge orifice groups are not driven in the same column position during the same scan (i.e., so controlled that nozzles of different discharge orifice groups cannot be simultaneously driven), printing data supplied to the head for each column can be divided into discharge orifice groups, and a printing data transfer signal line can be shared by different discharge orifice groups. This makes it possible to reduce the costs of the printhead and printing apparatus.
Accordingly, in a conventionally proposed printing apparatus which scans a printhead having different nozzle groups, different discharge orifice groups are driven at different driving timings, thereby sequentially switching different discharge orifices.
FIGS. 10A to 10F are views showing various arrangements of discharge orifice groups of printheads used in such a printing apparatus. In FIGS. 10A to 10F, discharge orifices indicated by A and B form different discharge orifice groups, and the discharge orifice groups A and B cannot be simultaneously driven in this embodiment.
FIG. 10A shows an arrangement in which the discharge orifice groups A and B are formed by different discharge orifice rows (nozzle rows), and these two rows are shifted from each other by the half nozzle pitch. FIG. 10B shows an arrangement in which the discharge orifice groups A and B are alternately arranged in the same row. FIG. 10C shows an arrangement in which two rows of each of the discharge orifice groups A and B are formed, and these two rows of each discharge orifice group are shifted from each other by the half nozzle pitch.
FIGS. 10D to 10F illustrate arrangements in each of which discharge orifice groups different in discharge amount are formed for one printing ink. That is, in these arrangements shown in FIGS. 10D to 10F, the discharge amounts of the discharge orifice groups A and B are different, i.e., the discharge amount of the discharge orifice group A is larger. In each of the arrangements shown in FIGS. 10D to 10F, two rows of each of the discharge orifice groups A and B are formed, and these two rows of each discharge orifice group are shifted from each other by the half nozzle pitch. However, these arrangements are different in row arrangement order. In the arrangement shown in FIG. 10F, two rows in each of which the discharge orifice groups A and B are alternately arranged are formed, and the positions (the order in the row) of discharge orifices indicated by A and B in one row are different from those of the other row.
When printing is to be performed by using a printhead having discharge orifice groups different in discharge amount, nozzles having a small discharge amount are used for highlighted portions to reduce the graininess, and nozzles having a large discharge amount are used for high-density portions to reduce the number of times of discharge and express high densities. In this way, the printing quality can be improved without lowering the printing speed.
In addition, when a printing apparatus which prints by using the printhead as described above has printing modes such as a printing mode (high-speed mode) in which images are formed by using only nozzles having a large discharge amount in order to give priority to the printing speed over the printing quality, and a printing mode (high-quality mode) in which images are formed by using only nozzles having a small discharge amount in order to give priority to the printing quality over the printing speed, printing meeting conditions desired by the user can be performed. This apparatus is disclosed in, e.g., Japanese Patent Laid-Open No. 8-183179.
The problem of a printing apparatus using a printhead having a plurality of discharge orifice groups as described above will be explained below by taking as an example a printhead having a plurality of discharge orifices different in discharge characteristic shown in FIG. 5. Referring to FIG. 5, nozzles having a large discharge amount are represented by “LARGE NOZZLE”, and nozzles having a small discharge amount are represented by “SMALL NOZZLE”. The same applies to the following explanation.
The printing position adjustment performed for this printhead having nozzles different in discharge amount as described above is based on the assumption that the driving timings of the large and small nozzles are different when printing is performed by the same scan.
FIGS. 6, 7, 30A, and 30B are views for explaining the discharge operation and the positions of printed dots when the printing resolution of the printhead shown in FIG. 5 is 600 dpi and the printing position adjustment pitch is 1,200 dpi.
Referring to FIGS. 30A and 30B, the abscissa indicates the main scan direction, and a printhead 701 can be driven to discharge ink in each column position indicated by the alternate long and short dashed line. The printhead 701 drives a discharge orifice group 701A (large nozzles) and a discharge orifice group 701B (small nozzles) at different driving timings during the same scan, thereby printing a target pixel 700.
FIG. 30A shows the state in which the discharge orifice group 701A (large nozzles) is driven in main scan direction printing position 0. FIG. 30B shows the state in which, after the state shown in FIG. 30A, the printhead 701 is moved by 1,200 dpi to the left in FIG. 30B and the discharge orifice group 701B is driven in main scan direction printing position 1. Even when the discharge orifice groups 701A and 701B are driven at these timings, dots are printed in a 1,200-dpi position on the left side of the target pixel 700 (a 600-dpi pixel including main scan direction printing positions 2 and 3) because the ink discharge speed and discharge direction of one discharge orifice group are different from those of the other.
In each of FIGS. 30A and 30B, the scan direction of the printhead 701 is indicated by the arrow, and. a discharge orifice group (nozzle group) currently being driven in the printhead 701 is hatched. FIG. 30A indicates that the large nozzle row 701A is driven, and FIG. 30B indicates that the small nozzle row 701B is driven. The dots printed in the target pixel 700 by the above driving are hatched in the target pixel 700. For convenience's sake, the sizes of these printed dots in the target pixel are the same as the sizes of the respective corresponding discharge orifices, and the relationship between the nozzle which has used to print the dot and the dot printed in the target pixel is indicated by the arrow. In FIG. 30B, the position of the printhead when the large nozzles are driven in FIG. 30A is also indicated by the dotted lines.
FIG. 6 shows FIGS. 30A and 30B in the same drawing. Referring to FIG. 6, the positions of the printhead at driving timings at which ink droplets discharged from the individual discharge orifice groups can be printed in the target pixel 700, when the discharge directions and discharge speeds of these ink droplets are taken into account, are illustrated above and below the target pixel 700. The relationships between the discharge orifice groups used and the printed dots are indicated by the arrows. In the following description, the two printing states of the printhead during printing scan in the main scan direction are illustrated in one drawing as shown in FIG. 6.
FIG. 6 shows the state in which when the driving timings of the large nozzles 701A and small nozzles 701B are staggered by 1,200 dpi, ink droplets discharged from the large and small nozzles can be printed in the same column position of the target pixel 700. FIG. 7 shows the state in which when the driving timings of the large nozzles 701A and small nozzles 701B are the same, ink droplets discharged from the large and small nozzles can be printed in the same column position of the target pixel 700.
When ink droplets are to be printed in the same column position as shown in FIG. 6, no problem arises under conditions by which the individual discharge orifice groups are driven at different timings (in different column positions). However, when ink droplets cannot be printed in the same main scan direction printing position (column position) unless the driving timings of the large and small nozzles are the same as shown in FIG. 7, the printhead based on the assumption that the large and small nozzles are driven at different timings as mentioned earlier cannot print dots in the same column position.
Note that the above-mentioned discharge orifice groups having different characteristics are not only nozzle groups having different discharge amounts, but also nozzle groups used to print dots different in density. Examples are discharge orifice groups which discharge ink droplets of the same color but different in density, and discharge orifice nozzles which discharge ink droplets of different colors to perform color printing by using ink liquids of a plurality of colors. Also, the aforementioned problem similarly arises in a printhead which includes different discharge orifice groups having the same characteristics, and which is so restricted as to be unable to drive these discharge orifice groups in the same column position (at the same timing).